The invention relates to combinations of substances that can be used as vapor-phase corrosion inhibitors (volatile corrosion inhibitors, VCIs) in particular to protect non-ferrous metals or metals that cannot be passivated, like copper, silver, manganese, magnesium and their alloys, against atmospheric corrosion.
It is already generally known that corrosion inhibitors which in powder form tend to sublimate under normal conditions, and through which vapor phases can penetrate to metal surfaces that are to be protected, are used for temporary corrosion protection of metal objects in closed areas, eg in packaging or showcases.
As a rule these vapor-phase inhibitors (VPIs) or volatile corrosion inhibitors (VCIs) are selected according to the type of metal to be protected and used as powder, packed in bags of a material that is permeable to the vaporous VPIs (cf H. H. Uhlig, Corrosion and Corrosion Protection (German), Akademie-Verlag Berlin, 1970, pp 247-249; K. Barton, Protection against Atmospheric Corrosionxe2x80x94Theory and Practice (German), Verlag Chemie, Weinheim, 1973, p 96 ff; I. L. Rozenfeld, Corrosion Inhibitors (Russian), Izt-vo Chimija, Moscow, 1977, p 320 ff).
Modern means of packaging for corrosion protection contain VCIs either in pellet form in porous foam plastic capsules or as fine powder inside polymeric carrier materials. Thus, in the patents U.S. Pat. Nos. 3,836,077, 3,967,926, 5,332,525, 5,393,457, 4,124,549, 4,290,912, 5,209,869, JP 4,124,549, EP 0.639.657 and DE-OS 3.545.473, different variants are suggested for incorporating VCIs in pellets or plastic films permeable to air, either by enclosing them in cavities created by opening up foam plastic and then covering them by a material permeable to gas or by adding the VCIs to the polymer melt intended for blow extrusion or injection molding so that a packaging material (film or hard substance) results from which the VCI components can continuously sublimate because of the structural porosity.
Attempts have already been made to work in the VCIs during the expansion of polymer solids, as described for example in JP 58.063.732, U.S. Pat. No. 4,275,835 and DD 295.668. Furthermore, packaging containing VCIs can be produced by dissolving the VCI components in a suitable solvent and depositing them on the particular packaging material. Methods of this kind with different active components and solvents are described for example in JP 61.227.188, JP 62.063.686, JP 63.028.888, JP 63.183.182, JP 63.210.285, DE-PO 1521900 and U.S. Pat. No. 3,887,481.
But, given the fact that the VCI packaging material produced in this way usually contains the active components only loosely in the structural cavities of the carrier material, paper, cardboard and foam plastic, etc, there is danger of mechanical spreading and spilling of the active particles so that it is not possible to ensure that the pretreated carrier materials at all possess the necessary specific surface concentration of VCIs when they are used for corrosion protection.
To eliminate this drawback, DE-PS 19708285 describes a corrosion-inhibiting composite material that consists of a mixture of a metallic oxide sol, the corrosion inhibitors capable of sublimation and further additives and forms a firmly adhering, sufficiently porous gel film of the used metal oxides and additives on the carrier material from which the corrosion inhibitors (VCIs) are released at a steady, long lasting rate of emission.
According to the ISO definition, a corrosion inhibitor is a xe2x80x9cchemical substance which decreases the corrosion rate when present in the corrosion system at a suitable concentration without significantly changing the concentration of any other corrosive agent; the use of the term inhibitor should be qualified by the nature of the metal and the environment in which it is effectivexe2x80x9d (see xe2x80x9cCorrosion of metals and alloysxe2x80x94Terms and definitionsxe2x80x9d, ISO 8044, 1986).
The major principle of using VCIs is to maintain or reinforce the inherent primary oxide layer, usually offering only limited protection, that forms very fast on every metal through contact with the atmosphere but cannot be perceived without optical aids (K. Barton, loc. cit.).
As regards the nature and properties of the mentioned primary oxide layer, the familiar commodity metals and their alloys can be divided into two categories, those where a sufficiently strong oxidizer is needed to maintain the protective primary oxide layer, and those where the passive oxide layer undergoes such chemical and/or structural changes through the action of an oxidizer that adhesion to the substrate and thus also the protective effect against corrosion are lost.
Among iron materials the primary oxide layer consists for the most part of an Fe (III) oxide. If the metal surface becomes damp, as is the case, for example, when a water film condenses, without the simultaneous action of a sufficiently strong oxidizer, then corrosion of the metal commences through transformation of these oxides into Fe (II) compounds, eg:
Fe2O3+H2O+2H++2exe2x88x92xe2x86x922Fe(OH)2
and where, for the anodic corrosion of the substrate metal:
Fe+2H2Oxe2x86x92Fe(OH)2+2H+2exe2x88x92
To avoid this, the action of a sufficiently strong oxidizer is necessary. Nitrites and, in particular, the relatively readily volatile dicyclohexyl ammonium nitrite have consequently been used for more than 50 years as vapor-phase inhibitors (cf Uhlig, Barton, Rozenfeld, loc. cit.) and are named as a constituent of VCI compositions in numerous patents (eg U.S. Pat. Nos. 2,419,327, 2,432,839, 2,432,840, 4,290,912, 4,973,448, JP 02085380, JP 62109987, JP 63210285 A, DE-PS 4040586).
The metals whose primary oxide layer is sensitive to further oxidation include e.g. copper, silver and manganese.
In Cu and Cu base materials the primary oxide film consists mainly of the oxide Cu2O for example. This film is only stable in hydrous media free of oxidizers, independently of the pH value. Exposed to the effect of oxygen, the oxide CuO is produced relatively fast, which is perceivable as a black deposit that, because of its crystal lattice dimensions, cannot intergrow with the metal substrate (no epitaxy) and therefore does not guard against corrosion. The initiating reactions of atmospheric corrosion can be formulated as follows: 
For the creation of VCI packaging means that cannot only be used for a certain kind of metal but that are also multivalent, it was attempted to formulate VCI combinations that contain not only amine nitrites but also components which are able to protect heterogeneous cast materials and precisely those metals like copper and silver base materials against corrosion.
In the course of this it was proposed to combine nitrites with further substances capable of sublimation, like the salts of medium-strength to weak, saturated or unsaturated carboxylic acids (cf U.S. Pat. No. 2,419,327, 2,432,839, 2,432,840, DE 814.725). Although this produces protection of the common Al, Sn and Zn materials, corrosion of Cu and Mg materials in the same packaging is further promoted.
The cause of this is found in the existence of nitrite, which cannot only oxidize the primary oxide layer of the copper but reduces to ammonia NH3 when acting as an oxidizer. This NH3 can, on the one hand, transform the oxidic passive layer of the copper metals into soluble complexes and, on the other, create such high alkalization on Mg surfaces when humidified that a soluble magnesium hydroxo complex is produced from the existing MgO film. In both cases this means a loss of the passive state and consequently the beginning of corrosion (cf A. D. Mercer, Proc. of the 7th Europ. Symp. on Corrosion Inhibitors, Ann. Univ. Ferrara/Italy, N. S., Sez. V, Suppl. N. 9 (1990), 449 pp).
To eliminate this drawback, VCI systems were proposed that are to be suitable for corrosion protection of any metal combinations but are to make do without nitrite and amines by being composed solely of combinations of organic carboxylic acids and their salts, as for example in DE-OS 877.086, CS-PS 124.738 and PL-PS 96.548. But in general this does not result in dependable corrosion protection, because the rate of sublimation is comparatively low and reduces even more with increasing relative humidity.
To eliminate this drawback, it was proposed to combine salts like benzoates with a readily volatile amine and thus accelerate the exit velocity of the VCIs from the packaging serving as a depot. Thus U.S. Pat. No. 4,124,549, for example, speaks of combinations of dicyclohexylamine with benzoic acid and/or caprylic acid as well as the analog combinations monoethanolamine laurate and morpholine benzoate, while DE-OS 3219360 claims use of morpholine caprylate. JP 61227188 specifies the salts of tertiary amines like dimethylethanolamine caprylate mixed with hexamethylene tetramine, while JP 09228078 considers cyclohexyl cyclohexamine, cyclohexyl benzenamine and other homologs that, furthermore, when dissolved in propanol can be deposited on paper as a carrier material or can even be used as a readily volatile corrosion-inhibiting fluid. Instead of the combination of carboxylic acid salts with amines, DE-OS 3210360 names mixtures of fatty alcohol phosphates with volatile amines as VCIs, eg mixtures of Di(2-ethylhexyl) hydrogen phosphate and Di(9-octadecenyl) hydrogen phosphate with morpholine or morpholine caprylate.
Practical uses of such systems have been lacking to date however, probably and chiefly because laboratory experiments proved that they do not produce reliable corrosion protection effects for multi-metal combinations, since the usual kinds of steel in particular cannot be guarded against corrosion if the VCI system does not contain any oxidizer functioning as a passivator, and, as a result of the relatively high alkalization produced by the named substances on metal surfaces in humid air, the primary oxide layers found on copper, aluminum, zinc and magnesium become more complex, thus contributing to canceling the passive state.
Furthermore, copper and silver materials are not protected against corrosion by these combinations of substances anyway if the usual impurities of industrial air, eg carbon dioxide, sulfur dioxide, hydrogen sulfide, are present at the same time in the atmosphere of the inner space of the particular packaging. These metals are after all characterized by the fact that they have a higher affinity to sulfidic sulfur than to oxygen, and the blackish brown tarnishing film on silver is ascribed to formation of the silver sulfide Ag2S.
Benzotriazole has long been in use for protecting copper and copper alloys against atmospheric corrosion (cf Barton, Mercer, loc. cit.). But, seeing as the tendency of this compound to sublimate is relatively low, DE-PS 1182503 and U.S. Pat. No. 3,295,917 suggest first setting the depot of this VCI to a higher temperature (up to approx. 85xc2x0 C.) and at the same time cooling the metal objects on which condensation is to occur. U.S. Pat. Nos. 2,941,953 and 3,887,481, on the other hand, describe the impregnation of paper with benzotriazole and/or tolyltriazole. Organic solvents like tetrachlorethylene are used and it is specified that the metal parts to be protected should be enclosed as tightly and densely as possible with the impregnated VCI packaging in order to keep the space between the VCI depot and the protected metal surface as small as possible. However, this technology exhibits the disadvantage, already mentioned, that the active component in the form of extremely fine powder particles does not adhere very well to the paper and can easily trickle down, so the corrosion protection properties of this packaging cannot be considered reliable.
The tendency of benzotriazole and tolyltriazole to sublimate from a VCI depot likewise increases if further solids in powder form are worked in that are capable of sublimation. For this purpose EP 0662527 speaks of mixtures of benzotriazole with cyclohexylamine benzoate and ethylamine benzoate or with anhydrous sodium molybdate and dicyclohexylamine nitrite, U.S. Pat. Nos. 4,051,066 and 4,275,835 describe mixtures of benzotriazole with ammonium and amine molybdates, amine benzoates and nitrates, U.S. Pat. No. 4,973,448 addresses mixtures of benzotriazole with organic carbonates, phosphates and amines, while JP 62063686 and JP 63210285 A detail mixtures of benzotriazole with alkali and amine salts of aromatic carboxylic acids.
Combinations of benzotriazole, tolyltriazole or methylbenzotriazole with other nitrogenous organic, volatile solids are described in JP 62109987, JP 61015988, DD-PS 268978 and DD-PS 298662 for example. A disadvantage is the fact that all components containing ammonium ions and amines, because of their more or less pronounced tendency to conjugate with metal ions, again reduce the protective effect of triazoles especially in the case of non-ferrous metals. Furthermore, the stated amines and ammonium compounds are highly hydrophilic. VCI depots containing such substances consequently tend very much to absorb water. Because of their hydrolysis this is usually followed by a stronger drop in their tendency to sublimate, meaning automatically a reduction in the corrosion protection effect.
For these reasons JP-PS 56122884 A, for example, suggests, as an alternative, dispensing with additives containing amines and only using triazoles. But, to avoid having to wait for the triazole to sublimate from the packaging functioning as a VCI depot and be adsorbed on the metal surface to be protected, it is proposed that these inhibitors, dissolved in a suitable halogenated hydrocarbon, be sprayed direct onto the metal parts from spray bottles. These spray fluids are recommended in JP-PS 56122884 A especially for corrosion protection of copper materials and other alloy materials in electronic systems and printed microelectronic circuits. This kind of application of corrosion inhibitors no longer makes use of the advantages of the principle of volatile corrosion inhibitors (VCIs) however, instead it presents the disadvantage that, in addition to the packaging (encapsulating) process for the electronic components, spraying makes an extra operation necessary.
In order to make use of the advantages of VCIs and the inhibitor effect of the triazole structure, JP-PS 03079781 suggests using only alkylamine triazoles instead of triazole/amine combinations. The explicitly stated substances 3-amino-1,2,4-triazole and 3-amino-5-methyl-1,2,4-triazole do in fact exhibit a higher rate of sublimation but, especially where copper and silver are concerned, not such a marked corrosion protection effect as benzotriazole and tolyltriazole.
As an alternative to the triazoles, DD 284255 quite globally speaks of the use of indole or imidazole derivatives to protect non-ferrous metals against corrosion. But there are neither details of the type and concentration of such additives, nor is their protective effect substantiated by concrete data.
It would therefore be advantageous, compared to the drawbacks of conventional corrosion inhibitors as described above, to provide improved corrosion-inhibiting substances and combinations of substances that are capable of sublimation and which, especially in the climatic conditions of practical interest inside technical packaging and analogous closed spaces, sublimate from a depot with sufficient speed and, after adsorption and/or condensation on the surface of metals located in such areas, produce conditions in which, in particular, non-ferrous metals like copper, silver, manganese, magnesium and their base alloys, rated as incapable of passivation, can be effectively protected against atmospheric corrosion. It would further be advantageous to provide methods for producing or processing such substances and combinations of substances for the manufacture of improved VCI packaging.
This invention relates to a corrosion-inhibiting combination of substances, capable of sublimation, that contains (1) aromatic mercaptothiazole or triazole, (2) multiple substituted phenol insoluble in water, (3) L ascorbic acid or one of its salts, and (4) a carboxylic acid/salt pair.
Surprisingly, the drawbacks of conventional corrosion inhibitors have been solved by the invention in particular by combinations of substances made up of the following components:
(1) aromatic mercaptothiazole or triazole,
(2) multiple substituted phenol insoluble in water, and
(3) L ascorbic acid or one of its salts.
Harmonized with the components (1) through (3), a suitable carboxylic acid/salt pair may possibly be added as component (4). The carboxylic acid/salt pair is chosen so that, after its sublimation and condensation on metal surfaces, the pH value of a condensed water film is stabilized (buffered) there in the range 4.8xe2x89xa6pHxe2x89xa66.5.
The invention foresees direct use of these combinations of substances in the form of appropriate powder mixtures or inclusion by already familiar methods as part of producing VCI packaging so that this packaging functions as a VCI depot and the corrosion protection properties of the combinations of substances according to the invention can show to special advantage.
The combinations of substances according to the invention are used above all to protect metals that are considered as being incapable to be passivated, eg copper, silver, manganese, magnesium and their alloys, in packaging and during storage in analogous closed spaces against atmospheric corrosion.
A subject of the invention is in particular a corrosion-inhibiting material consisting of a component that is an aromatic mercaptothiazole or triazole and is specifically adsorbed above all on surfaces of non-ferrous metals, a further component that is a multiple substituted phenol and, as a result of its properties of not being soluble in water but easily adsorbed by solids, hydrophobizes all other components of the combinations of substances according to the invention and, because of its relatively high sublimation pressure, transports them as a carrier material through the gas space to the metal surface to be protected, a component that is L ascorbic acid or one of its salts and, because of its property of working as an anti-oxidant, surprisingly inhibits the effect of atmospheric oxygen on metal surfaces and thus the corrosion process, and finally a suitable carboxylic acid/salt system that, in condensed water films on metal surfaces, stabilizes the pH value in the range 4.8xe2x89xa6pHxe2x89xa66.5 where the previously named components of the combinations of substances according to the invention can optimally demonstrate their corrosion protection effect. The combinations of substances according to the invention advantageously consist exclusively of non-toxic substances that are no danger to the environment and can be processed easily and without risk by already familiar methods. Consequently they are especially suitable for producing corrosion-protective packaging that is inexpensive on a large scale and can be used without posing any risk.
For inclusion of the combinations of substances according to the invention in VCI depots or in packaging functioning as such, it is best to first finely mill the individual substances down to particle sizes of xe2x89xa620 xcexcm, then thoroughly dry them and finally mix them as intensively as possible by familiar methods.
The combinations of substances according to the invention should be formulated in the following relations:
component (1): 10 to 40%
component (2): 10 to 40%
component (3): 10 to 40%
or if all four components are used:
component (1): 10 to 40%
component (2): 10 to 35%
component (3): 10 to 30%
component (4): 10 to 30%